1. Field of the Invention
The present invention relates a light emitting device that is capable of emitting light of various colors by combining a light emitting diode and fluorescent materials, and particularly to a light emitting device that has a lens for controlling the light distribution characteristic.
2. Background Art
In recent years, the development of the blue light emitting diode that utilizes nitride semiconductor has made it possible to manufacture light emitting devices that emit light of various colors by combining the blue light emitting diode and fluorescent materials which absorb at least part of light emitted by the light emitting diode and emit light of different color tones. A light emitting diode that emits white light, in particular, can be made by using a blue light emitting diode as the light emitting diode and a fluorescent material which absorbs part of light emitted by the blue light emitting diode and emits light of a color complementary to blue.
These light emitting devices can be classified into various types including bullet type and surface-mounted type.
A bullet type light emitting device has a cup formed at the distal end of one of positive and negative lead electrodes, and a light emitting diode mounted in the cup with the cup filled with a resin which includes a fluorescent material dispersed therein. The cup is then surrounded by a molding resin that is formed in a bullet shape having lens-shaped tip (refer to, for example, Japanese Unexamined Patent Publication (Kokai) No. 7-99345).
A surface-mounted type light emitting device has such a constitution as a recess is formed on a substrate having a positive electrode and a negative electrode formed thereon, so that a light emitting diode is mounted in the recess which is then filled with a resin including a fluorescent material dispersed therein (refer to, for example, Japanese Unexamined Patent Publication No. 2002-319711).
When white light is emitted by combining the blue light emitting diode and a fluorescent material, in particular, color tone of the white light is determined by the balance between the intensity of light emitted by the blue light emitting diode and the intensity of light emitted by the fluorescent material. However, since it is difficult to control the quantity of the fluorescent material dispersed in the resin which fills in the light emitting device, a problem of difference in color tone due to different amounts of fluorescent material among individual devices arises. To counter this problem, Japanese Unexamined Patent Publication (Kokai) No. 2001-177158 discloses a method of correcting the difference in color tone by grinding the resin layer that includes the fluorescent material thereby to control the quantity of the fluorescent material. Japanese Unexamined Patent Publication (Kokai) No. 2004-186488 discloses a method of correcting the difference in color tone by controlling the thickness of the resin layer in a portion which does not include the fluorescent material.
In the light emitting device that combines the light emitting diode and the fluorescent material, color unevenness often becomes a problem that the emitted light show different colors depending on the direction of view. The color unevenness occurs when the light rays pass through different amounts of fluorescent material depending on the different directions. In order to mitigate the color unevenness between different directions of view, therefore, the fluorescent material is preferably concentrated in the vicinity of the light emitting diode. For this purpose, such means have been taken as surrounding the light emitting diode with a cup that only is filled with a resin including a fluorescent material dispersed therein and covering the light emitting diode and the cup as a whole with a sealing resin layer formed in a lens shape (refer to, for example, Japanese Unexamined Patent Publication (Kokai) No. 10-242513), or dripping a resin that includes a fluorescent material dispersed therein only in a region surrounding the light emitting diode and, after curing the resin to harden, covering the light emitting diode and the cup as a whole with a sealing resin layer formed in a lens shape (refer to, for example, Japanese Unexamined Patent Publication (Kokai) No. 2000-315824).
It is also in practice to control the light distribution characteristic of a light emitting device that uses a light emitting diode, by forming a lens on a light transmitting sealing resin layer. The transparent sealing resin layer may be formed, for example, by the following methods.    (1) A sealing resin layer is formed into lens shape by molding.    (2) A sealing resin layer formed in a flat plate shape is formed into lens shape by machining.    (3) A lens formed beforehand is attached to the surface of a sealing resin layer.    (4) A casting case is used.
Among these, the method of forming the sealing resin layer into lens shape by molding is simple and suited to volume production, and is therefore widely employed. For molding the resin, it is common to the employ transfer mold process that is widely applied to the sealing resin layer for semiconductor chips (refer to, for example, Japanese Unexamined Patent Publication (Kokai) No. 2000-196000 and Japanese Unexamined Patent Publication (Kokai) No. 2001-352105).
In recent years, there is increasing demand for lower profile on side view type light emitting device, which is a kind of surface-mounted light emitting diode. The side view type light emitting device is a low-profile light emitting diode that emits light from a side face thereof. Many of the side view type light emitting device emit light from a side face that adjoins the mounting surface. There is also a demand for the surface mounted type light emitting device to form the resin layer that constitutes the light emitting surface into a lens shape, so as to provide a favorable light distribution characteristic. However, in case a lens is formed on the light emitting surface of the surface mounted type light emitting device, there have been such problems that the light emitting device becomes larger in size and requires a complex manufacturing process. When it is attempted to correct the difference in color tone by grinding the resin layer that constitutes the light emitting surface, as described in Japanese Unexamined Patent Publication (Kokai) No. 2001-177158 and Japanese Unexamined Patent Publication (Kokai) No. 2004-186488, there has been such a problem that the lens formed on the light emitting surface deforms, thus causing the light distribution characteristic to alter.